Electrolyte for electrochemical material removal



United States Patent 3,230,160 ELECTROLYTE FOR ELECTROCHEMICAL MATERIAL REMOVAL George C. Kennedy, Cincinnati, Ulric, assignor to General Electric Company, a corporation of New York No Drawing. Filed Sept. 19, 1962, Ser. No. 224,834 3 Claims. (Cl. 204-143) This invention relates to electrolyte solutions and, more particularly, to an electrolyte especially suitable for electromechanical material removal processes.

Methods and equipment for the electrochemical removal of material from a workpiece have-been known for a great number of years. With such method, which is the opposite of electrodeposition, and in the use of such equipment, the workpiece from which material is to 'be removed is the anode and a tool is the cathode. In the presence of a suitable electrolyte, current is passed between the cathode and the anode to bring about deplating of material of the anode-workpiece.

More recent forms of this basic procedure for removing material electrochemically rather than by means of an arc discharge have taken the form of general removal of surfaces or edges as shown by US. Patent 2,590,927--Brandt et al., the form of electrolytically cutting metal as shown by US. Patent 2,739,935Kehl et al., the form of electrolytic grinding processes and apparatus one example of which is shown in US. Patent 2,746,9l7- Comstock and the selective electrolytic material removal to form various types of cavities and holes as shown by patent application S.N. 823,975Crawford et al., filed June 30, 1959, and abandoned and which is assigned to the assignee of the present invention.

One of the essential elements in all of these processes is the electrolyte which conducts current between the two electrodes allowing material to be eroded electrochemically from the workpiece. In order to provide adequate conductivity in electrolytes, thus to lower the resistance to flow of current between the electrodes, a variety of salts, acids and the like are used. However, many electrolytes are highly corrosive to equipment, or produce intergranular attack on material of the workpiece. This is particularly true when the electrolyte conductivity is adjusted to provide optimum feed rates for the tool and workpiece one toward the other in order to achieve maximum material removal rates.

It is a principal object of this invention to provide an electrolyte capable of 1) protecting ordinary, commonly used metallic equipment from corrosion, (2) eliminating intergranular attack on a metallic workpiece and (3) operating at optimum material removal rates with regard to conductivity through the solution.

Another object is to provide an electrolyte which can be inexpensively and easily prepared, which does not become rancid and which is safe with regard to the health of workmen.

Briefly, the electrolyte of the present invention consists essentially of an aqueous solution of 90240 grams of sodium formate and about 20-80 grams of sodium nitrite per liter of water. The most effective and preferred form of the invention consists essentially of about 120 grams of sodium formate and about 26 grams of sodium nitrite per liter of water.

Of the two salts included in electrolyte of this invention, the sodium formate provides the principal amount of electrical conductivity while the sodium nitrite provides principally corrosion resistance. However both are salts which will ionize in water so that both will contribute some conductivity to the electrolyte.

As sodium formate is dissolved in water, the solutions resistivity decreases from about 20 ohms at about Patented Jan. 18, 1966 50 grams per liter to about 9 ohms at 400 grams per liter. As sodium formate and sodium nitrite are combined in an aqueous solution, the resistivity further decreases so that at 50 grams per liter of sodium formate and about 175 grams per liter of sodium nitrite, the specific resistivity of an aqueous solution is between 7 and 7.5 ohms. This same condition exists up to a concentration of about 300 grams per liter of sodium formate with the same amount of sodium nitrite. As sodium nitrite is added to a solution ranging between about 50 and 300 grams per liter of sodium formate, the specific resistivity of the solution progressively decreases until at about 400 grams per liter of sodium nitrite the specific resistivity is between 5 and 5.5.

The effect of sodium nitrite in water as a corrosion inhibiting agent is well known. Therefore, in order to provide a combination of corrosion resistance for equipment and workpiece from the effect of sodium nitrite as well as low specific resistivity and hence high conductivity, as provided by the combination of sodium nitrite and sodium formate, it would seem that substantial amounts of sodium nitrite would be more appropriate. However, it has been found that certain amounts of sodium nitrite are detrimental to the workpiece by causing intergranular attack. It is believed that such effect is partly due to the existence of stray current as a result of the lower resistivity of the solutions including greater amounts of sodium nitrite. Thus, a well balanced electrolyte for electrochemical material removal need not necessarily have the highest conductivity.

A chart of aqueous solutions of the two salts sodium formate and sodium nitrite show specific resistivity areas which might appear more favorable than that of the present invention. However, it has been found that the particular composition of 120-240 grams of sodium formate and 2570 grams of sodium nitrite per liter of water, unexpectedly provides an electrolyte having an overall balance of (1) relatively low specific resistivity, (2) resistance to corrosion of equipment and (3) resistance to intergranular attack on workpiece material. It has been found that the conductivity of aqueous electrolyte solutions including less than about grams of sodium formate and sodium nitrite up to about 80 grams per liter of water have excessively high specific resistivity. Therefore it is less desirable and less practical an electrolyte.

The data of the following Table I is typical of a wide variety of solutions studied in connection with the present invention. As shown :by Examples 1, 2 and 3, the range of 2570 grams of sodium nitrite and -240 grams of sodium formate per liter of water is specifically preferred with regard to pitting of the specimen. Examples 4 and 5 show that sodium nitrite alone or in the presence of too much sodium formate will not prevent pitting of the specimen. In addition, Example 6 shows that a large amount of sodium nitrite can do more harm from a pitting corrosion point of view than can the relatively large amount of sodium formate in Example 5.

TABLE I Aqueous solutions Example NaNOz (5/ HCOONa Specimen (al Condition The most desirable amount of sodium formate in the electrolyte of the present invention is at about 1 pound per gallon of water (about 120 grams per liter of Water) in the presence of a particular amount of sodium nitrite. The data of Table II shows that with such an amount of sodium formate, less than about 20 grams per liter of sodium nitrite will result in a large amount of corrosion, with about 20 grams per liter representing a point at which only slight corrosion of the specimen takes place. It Was found that between about 25 and 80 grams of sodium nitrite per liter of Water in such a solution did not corrode the specimen Whereas about 80 grams per liter resulted in the specimens surface being pitted.

TABLE II Aqueous solution with 120 g./l. of HCOONa NaNo (g./l.): Results 16 Very corroded. 20 Slightly corroded. 25 No corrosion. 80 Surface pitted.

Thus it can be seen that an unusual combination of resistance to surface corrosion and resistance to intergranular attack, along with proper conductivity, can be achieved in an electrolyte within the range of the present invention. This is particularly true of an electrolyte consisting essentially of, 25-70 grams of sodium nitrite and 120-240 grams of sodium formate per liter of water. From an economical and health point of view along with optimum material removal rates, it has been found that an electrolyte including about 25 grams of sodium nitrite and about 120 grams of sodium formate for each liter of water is particularly desirable.

An electrolyte consisting of sodium chloride and water has been used for electrochemical machining processes where corrosion and pitting is not a problem. However, it has been found that solutions of sodium chloride and sodium nitrite in a wide variety of proportions does not inhibit severe surface pitting of the workpiece or equipment subject to such corrosion with or without various amounts of sodium formate.

Although this invention has been described in connection with certain specific examples, these are not meant to be limitations on the scope of the invention. Those skilled in the art of electrolytes and electrochemical material removal will recognize the variations and modifications of which the invention is capable.

What is claimed is:

1. An aqueous solution, suitable for use as an electrolyte in electrochemical material removal, consisting of sodium formate, sodium nitrite and water, the sodium formate content being -240 grams per liter of water and the sodium nitrite content being 20-80 grams per liter of water.

2. An aqueous solution, suitable for use as an electrolyte in electrochemical material removal, consisting of sodium formate, sodium nitrite and water, the sodium formate content being -240 grams per liter of water and the sodium nitrite content being 25-70 grams per liter of water.

3. An aqueous solution, suitable for use as an electrolyte in electrochemical material removal, consisting of sodium formate, sodium nitrite and water, the sodium formate content being about 120 grams per liter of water and the sodium nitrite content being about 25 grams per liter of water.

References Cited by the Examiner UNITED STATES PATENTS 2,939,825 6/1960 Faust et a1. 204142 3,004,910 10/1961 Keeleric 204-143 3,046,206 7/1962 Johnson et a1. 204-l43 JOHN H. MACK, Primary Examiner. 

1. AN AQUEOUS SOLUTION, SUITABLE FOR USE AS AN ELECTROLYTE IN ELECTROCHEMICAL MATERIAL REMOVAL, CONSISTING OF SODIUM FORMATE, SODIUM NITRITE AND WATER, THE SODIUM FORMATE CONTENT BEING 90-240 GRAMS PER LITER OF WATER NAD THE SODIUM NITRITE CONTENT BEING 20-80 GRAMS PER LITER OF WATER. 